Why is algae growing on my roof?
Algae feeds on the limestone filler in asphalt shingles. Because it is grasping onto the shingles it will not wash away over time. If fact, it will continue to spread and darken until removed.
Growth of algae on roofs has been most widespread in the Gulf States and along the Northwest and Eastern Seaboards. It is not, however, confined exclusively to these regions. Algae growth occurs to varying degrees in all regions of the country, especially those with warm, humid conditions.
Almost all types of roofing systems are susceptible to algae discoloration. It is, of course, most readily visible upon light colored roofs.